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Cytochrome c orientation in electron transfer complexes with photosynthetic reaction centers of rhodobacter sphaeroides and when bound to the surface of negatively charged membranes characterization by optical linear dichroism



Cytochrome c orientation in electron transfer complexes with photosynthetic reaction centers of rhodobacter sphaeroides and when bound to the surface of negatively charged membranes characterization by optical linear dichroism



Biochemistry 26(2): 397-410



Heme orientation with respect to the membrane normal has been measured for the cytochromes c and c2 bound to photosynthetic reaction centers from Rhodobacter (Rhodopseudomonas) sphaeroides R-26 in reconstituted phosphatidylcholine vesicles. Previous kinetic studies have suggested that each cytochrome may bind in two configurations, which lead to either rapid or slow electron transfer to the flash-oxidized reaction center bacteriochlorophyll dimer. The rapid oxidation of cytochrome c is .apprx. 20-fold slower than that of cytochrome c2. Optical linear dichroism measurements reported here show that, for both cytochromes, only the population undergoing rapid oxidation is dichroic. A stoichiometry of 0.5 dichroic cytochrome c or c2 is found bound per reaction center. Prominent differences between the dichroism of the cytochrome c2-reaction center complex and that of the cytochrome c-reaction center complex show that heme orientation differs in the two cases. The dichroism of cytochrome c bound to the reaction center can be distinguished from its dichroism when bound to the surface of negatively charged membranes. Analysis of the dichroism spectra suggests that, for cytochrome c2, the heme is tilted 7.degree.-8.degree. closer to the membrane normal and rotated by 32.degree. compared to the cytochrome c-reaction center complex. The dichroism spectra are consistent with the notion that the site on the cytochrome c surface that binds to the reaction center is the same site that binds to mammalian cytochrome c oxidase and reductase. However, a different locus is implicated on the surface of cytochrome c2. These data suggest that although the tertiary structures of the cytochromes are homologous, the binding site is not conserved. These differences in cytochrome orientations may be in part responsible for the differences in the rate of electron transfer to the reaction center.

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Accession: 005084671

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DOI: 10.1021/bi00376a010


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